1. Field of the Invention
The present invention relates to an optical film having diffusibility. In particular, it relates to an optical film utilized as a diffusive sheet for a subsurface illuminator of a liquid crystal display.
2. Prior Art
The light source shown in FIG. 4 has been used as a surface light source for a subsurface illuminator of a lap-top type or note-type personal computer, a word-processor, a monochrome or colored liquid crystal display or the like.
In this light source, called an edge-light type light source, incident light from a tubular light source 30 is introduced to the side of a light conducting plate 20 having a light scattering pattern and illuminates the light conducting plate in accord with the pattern. A light diffusion sheet 10 is placed on the light emitting surface of the light conducting plate to hide the pattern.
The light diffusion sheet 10 is required to have good light transmittance, to hide the light scattering pattern of the light conducting plate, to have a good color rendering effect and the like. As the light diffusion sheet, there is known a plate formed by dispersing an inorganic or organic light diffusive agent in a resin such as methacrylic resin, polycarbonate resin, polyvinyl chloride resin or the like, or a thin film of polyester or the like to which a transparent resin solution including a light diffusive agent is applied.
Further, some color liquid crystal displays, which require a high degree of brightness, are provided with a light source equipped with a combination of the light diffusion sheet and a prism lens which directs as much of the light emitted from the light conducting plate as possible in the front direction. The light diffusion sheet of such a surface light source is required to have high light transmittance because the prism lens and the light diffusion sheet are stacked on the light emitting surface of the light conducting plate. However, the light diffusion sheet dispersed with a light diffusive agent contains approximately 150% by weight of the light diffusive agent based on the 100% by weight of the resin component and does not have high transmittance.
Therefore, as a light diffusion sheet for use in applications requiring high transmittance, one consisting of a resin having high transmittance such as polycarbonate and made light diffusive by surface embossing is adapted.
However, a metal embossing roller is necessary for producing the light diffusion sheet, which raises the manufacturing cost. Further, once a specific metallic roller has been fabricated, the roughness of the embossing can not be adjusted easily. In order to prepare light diffusion sheets having various kinds of light diffusibility, therefore expensive metal rollers corresponding to the required diffusibilities must be prepared. In addition, the surface of a high-transmittance resin, e.g., polycarbonate, generally exhibits troublesome glare.
An object of the present invention is therefore to provide an optical film which has high light transmittance and excellent light diffusibility. Another object of the present invention is to provide an optical film which exhibits desired light diffusibility by addition of a small amount of a light diffusive agent. Another object of the present invention is to provide an optical film which can be prepared easily and at a low cost.